KR20210019519A - Ferritic stainless steel sheet and its manufacturing method - Google Patents

Abstract

In addition to the predetermined component composition, the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more is 3.0 µm or more, and the minimum value of the maximum logarithmic deformation of the molding limit based on the molding limit line is 0.20 or more.

Description

Ferritic stainless steel sheet and its manufacturing method

[0001] The present invention relates to a ferritic stainless steel sheet having sufficient corrosion resistance and excellent formability, particularly, an extended formability, and a method for producing the same.

SUS430 (16 to 18 mass% Cr) ferritic stainless steel sheet is economical and excellent in corrosion resistance, so it is applied to various uses such as building materials, transportation equipment, home appliances, kitchen equipment and automobile parts. The scope of application has recently been further expanded.

Steel sheets applied to these applications are required not only to have corrosion resistance, but also to have sufficient formability that can be processed into a predetermined shape by press forming or the like.

As such a ferritic stainless steel sheet, for example, in Patent Document 1,

"By mass%, C: 0.02 to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11 to 30% or less, Ni: 0.7% or less is contained, and N is contained so as to satisfy 0.06≦(C+N)≦0.12 and 1≦N/C in relation to the C content, and further V Ferritic stainless steel sheet with excellent formability, characterized in that it contains 1.5 × 10 ^-3 ≤ (V × N) ≤ 1.5 × 10 ^-2 in relation to the N content, and consists of the balance Fe and inevitable impurities .」

Is disclosed.

In addition, in Patent Document 2,

"In mass%, C: 0.010 to 0.045%, N: 0.01 to 0.05%, Mn: 1% or less, Cr: 13 to 20%, Al: 0.01% or less, and C and N are contained in Cr carbonitride. It contains so that the volume ratio v is 0.09% or less, further contains Si: 0.4% or less, P: 0.05% or less, S: 0.010% or less, has a composition consisting of the balance Fe and unavoidable impurities, and further ferrite grains A ferritic stainless steel cold-rolled steel sheet excellent in press formability, characterized by having an average crystal grain size of 10 µm or more and a single ferrite structure in which 50 or less Cr carbonitrides are dispersed per ferrite grain.”

Is disclosed.

Japanese Patent Publication No. 3584881 Japanese Patent Publication No. 4682806 Japanese Patent Publication No. 58843211

By the way, press molding is largely divided into four types of molding modes such as elongation molding, deep drawing molding, extension flange molding, and bending molding.

In recent years, the molding mode in press molding is mainly a member that is elongated, for example, an exterior member such as an outdoor hood of an annular louver used for an exhaust duct or a ventilation port, and a design by embossing. Alternatively, ferritic stainless steel is being applied to an interior panel member with improved functionality. For this reason, development of a ferritic stainless steel sheet having excellent elongation moldability capable of processing into such a member shape is desired.

However, it could not be said that the ferritic stainless steel sheet disclosed in Patent Documents 1 and 2 had sufficient elongation formability.

So, the inventors first, in Patent Document 3,

"By mass%, C: 0.005 to 0.025%, Si: 0.02 to 0.50%, Mn: 0.55 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.10%, Cr: 15.5 to 18.0 %, Ni: 0.1 to 1.0%, N: 0.005 to 0.025%, the balance consists of Fe and inevitable impurities, the elongation at break is 28% or more, the average r value is 0.75 or more, and also FLD (molding limit line Fig.) A ferritic stainless steel sheet with a minimum value of 0.15 or more of the maximum logarithmic deformation of the forming limit based on.

Was developed.

Thereby, compared to the ferritic stainless steel sheet disclosed in Patent Documents 1 and 2, a ferritic stainless steel sheet having significantly improved elongation formability has been obtained.

However, when the ferritic stainless steel sheet of Patent Document 3 is to be formed into a member requiring particularly high elongation formability such as an exhaust duct, further cracking may occur, and therefore, further improvement in elongation formability is required. It is the current situation.

An object of the present invention is to provide a ferritic stainless steel sheet having sufficient corrosion resistance and excellent elongation formability, along with an advantageous manufacturing method, as developed in view of the above-described current situation.

Here, "sufficient corrosion resistance" refers to the salt spray cycle test specified in JIS H 8502, salt spray (35° C., 5 mass% NaCl, spray time: 2 hours) → drying (60° C., 40% relative humidity, holding time) : 4 hours) → rate of rust occurrence area on the steel plate surface when wet (50°C, relative humidity ≥ 95%, holding time: 2 hours) was carried out for 8 cycles as 1 cycle (rust occurrence area on the steel plate surface/ It means that the total area of the surface of the steel sheet) x 100 (%)) is 25% or less.

In addition, ``excellent elongation formability'' means that the minimum value of the maximum logarithmic deformation of the molding limit determined based on the forming limit diagram (Forming Limit Diagram, hereinafter also referred to as FLD) measured in accordance with ISO12004-2:2008 is 0.20 or more. Means that.

Therefore, the inventors repeated various examinations in order to solve the above-described problem.

First, the inventors prepared various ferritic stainless steel sheets with different component compositions and manufacturing methods, and using these steel sheets, a press working test on a member including a part that is biaxially extended and unequal biaxially extended. Was carried out.

In general, it is considered that the higher elongation is superior to the elongation in elongation, but in this press working test, even a steel sheet with a high elongation at break may cause cracks, and from this test result, the superiority in elongation formability is necessarily fractured. It was found that the size of the kidney alone was not determined.

Therefore, the inventors separately prepared a steel plate in which cracks occurred in the previous test, and then again performed a press working test under the same conditions using the steel plate, and press-fit of the upper mold where cracks occurred in the previous test. Immediately before the end position (bottom dead center + 2 mm), the press working was stopped, a test piece was collected from the steel sheet, and the metal structure was observed in detail. Specifically, after the above-described press working was stopped, the steel sheet was taken out from the mold, and the end face of the steel sheet was then mirror-polished, followed by corrosion treatment with saturated picric acid-5% by mass hydrochloric acid aqueous solution. Then, a test piece for metal structure observation was prepared, and the test piece was observed at 500 times magnification with a scanning electron microscope (secondary electron image).

As a result, in all steel sheets with cracks in the previous test, a large amount of voids have already been created at the interface of the Cr-based carbonitride and ferrite matrix in the middle of the press working, and some voids are connected with the nearby voids. It was confirmed that it was growing in microcracks.

On the other hand, in the metal structure after press working of a steel sheet that could be pressed without cracking in the previous test, voids were formed at the interface of the Cr-based carbonitride and ferrite matrix, but the occurrence of micro-cracks due to the connection between the voids Not confirmed.

In view of the above, the inventors believe that the superiority of the elongation formability is largely influenced by the metal structure of the steel sheet, and in the previous press test, the press working of both the cracked steel sheet and the steel sheet that could be formed without cracking The previous metal structure was investigated, and detailed comparison of both was performed.

As a result, both of the metal structures were ferrite structures in which Cr-based carbonitrides were dispersed, but it was found that the distance between Cr-based carbonitrides tended to be relatively short in the steel sheet in which cracks occurred in the previous press working test.

Therefore, the inventors repeatedly experimented and reviewed, paying attention to the relationship between the lengthening formability and the distance between the Cr-based carbonitride. As a result, it was confirmed that there was a correlation between the lengthening formability and the average distance between Cr-based carbonitrides of a certain size or more. Particularly, by setting the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more to 3.0 µm or more, excellent elongation moldability was obtained. Specifically, excellent elongation moldability was obtained in which the minimum value of the maximum logarithmic deformation of the molding limit determined based on the molding limit line diagram FLD was 0.20 or more. Thereby, the knowledge was obtained that a member requiring particularly high elongation formability such as an exhaust duct can be press-formed without cracking.

Here, by increasing the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more, the inventors consider the following reason for obtaining excellent elongation formability.

That is, in the case of processing a steel sheet, voids are generated at the interface between the ferrite matrix and the Cr-based carbonitride in the metal structure as the amount of deformation increases. This void increases and grows with an increase in the amount of deformation and/or an increase in the stress concentration, and becomes a crack by being connected with other adjacent voids, finally leading to fracture of the steel sheet.

In this way, the void grows by receiving stress concentration along with the increase in the amount of deformation, and grows into microcracks by being connected to the voids in the vicinity. In particular, in the deformation under the multiaxial stress in which the stress acts two-dimensionally or three-dimensionally, the degree of triaxial stress increases, thereby further promoting crack growth. In the case of such deformation under multiaxial stress, as compared with deformation under uniaxial stress (deformation under uniaxial stress is used for elongation evaluation and is represented by a tensile test), voids tend to grow. Therefore, it is considered that the fracture limit of the material becomes lower (that is, it becomes easier to fracture) than under uniaxial stress.

Longitudinal molding is usually a deformation under multiaxial stress, and since all-round voids are easily connected in a steel sheet, fracture is more likely to occur than deformation under uniaxial stress.

For this reason, even for a steel sheet that exhibits high breaking elongation in deformation under uniaxial stress such as a tensile test, if the average distance between Cr-based carbonitrides of a certain size or more is short, voids are connected in deformation under multiaxial stress, and voids are connected. The occurrence of micro-cracks resulting from and its progress is encouraged.

On the other hand, if the average distance of Cr-based carbonitrides of a certain size or more is sufficiently long, the connection of voids is difficult to occur even in the case of performing elongation molding that deforms under multiaxial stress. The occurrence of cracks and their progress are suppressed.

For this reason, the inventors believe that by increasing the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more, the elongation formability is significantly improved.

In addition, as a result of further investigations by the inventors, in order to make the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more to be 3.0 µm or more, hot-rolled sheet annealing was performed in a predetermined temperature range for a certain period of time or more. , The metal structure after the hot-rolled sheet annealing is once a ferrite single-phase structure in which Cr-based carbonitride is deposited, and then in the cold-rolled sheet annealing after cold rolling,

(1) By slowing the heating rate from 500°C to the heating temperature, coagulation and coarsening of Cr-based carbonitrides, and solid solution of Cr-based carbonitrides to the ferrite phase are simultaneously promoted. The solid solution in the ferrite phase is a phenomenon in which Cr-based carbonitride is decomposed into Cr, carbon, and nitrogen in atomic units, and each element is contained in the ferrite phase),

(2) by appropriately controlling the heating temperature and holding time to further promote the solid solution of Cr-based carbonitride to the ferrite phase, and,

(3) Re-precipitation of solid-solution Cr-based carbonitride is suppressed by increasing the cooling rate from heating temperature to 500°C.

This is important, and by satisfying all of these conditions at the same time, it was found that the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more can be 3.0 µm or more.

The present invention was completed after further examination based on the above findings.

That is, the gist configuration of the present invention is as follows.

1. By mass%,

C: 0.025 to 0.050%,

Si: 0.10 to 0.40%,

Mn: 0.45 to 1.00%,

P: 0.04% or less,

S: 0.010% or less,

Cr: 16.0 to 18.0%,

Al: 0.001 to 0.010%,

N: 0.025 to 0.060% and

Ni: 0.05 to 0.60%

And the balance has a component composition consisting of Fe and unavoidable impurities,

The average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more is 3.0 µm or more,

Ferritic stainless steel sheet with a minimum value of 0.20 or more of the maximum logarithmic deformation of the forming limit based on the forming limit diagram.

2. A steel material having the component composition described in the above 1 is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to annealing of the hot-rolled sheet at a heating temperature of 800 to 900°C and a holding time of 1 hour or longer, and then cold-rolled to cold rolled. The cold-rolled steel sheet was then annealed to the cold-rolled steel sheet at a heating temperature of 800 to 900°C and a holding time of 5 to 300 seconds.

In the above cold-rolled sheet annealing, ferrite, wherein the average heating rate at 500°C to the heating temperature is 20°C/s or less, and the average cooling rate at the heating temperature to 500°C is 10°C/s or more. Method of manufacturing a series stainless steel plate.

According to the present invention, a ferritic stainless steel sheet having sufficient corrosion resistance and excellent in elongation formability can be obtained.

Further, when the ferritic stainless steel sheet of the present invention is used, a member requiring particularly high elongation formability, such as an exhaust duct, can be manufactured by press molding, which is very advantageous industrially.

1 is a photograph of a metal structure of No. 1 in Examples.
Fig. 2 is a photograph of a metal structure of No. 12 in Example.

Hereinafter, the present invention will be described in detail.

First, the component composition of the ferritic stainless steel sheet of the present invention will be described. In addition, the units in the component composition are all "% by mass", but hereinafter, unless otherwise specified, they are simply expressed as "%".

C: 0.025 to 0.050%

C is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of leasing. From the viewpoint of obtaining such an effect, the C content is set to 0.025% or more.

However, when the C content exceeds 0.050%, the amount of precipitation of Cr-based carbonitrides during hot rolling and hot-rolled sheet annealing becomes excessively large, making it difficult to lengthen the average distance between Cr-based carbonitrides. For this reason, it is not possible to prevent the occurrence of cracks caused by connection of voids and breakage due to propagation during extension molding, and desired extension moldability cannot be obtained. Further, the steel is excessively hardened and the ductility is lowered.

Therefore, the C content is in the range of 0.025 to 0.050%. The lower limit of the C content is preferably 0.030%, more preferably 0.035%. Moreover, the upper limit of the C content is preferably 0.045%.

Si: 0.10 to 0.40%

Si is an element that acts as a deoxidizing agent in steel solvents. From the viewpoint of obtaining such an effect, the Si content is made 0.10% or more.

However, when the Si content exceeds 0.40%, the steel is excessively hardened and the rolling load at the time of hot rolling increases. Further, the ductility of the steel sheet obtained after annealing of the cold-rolled sheet decreases.

Therefore, the Si content is in the range of 0.10 to 0.40%. The lower limit of the Si content is preferably 0.20%. The upper limit of the Si content is preferably 0.30%.

Mn: 0.45 to 1.00%

Mn, like C, is an element effective in promoting the formation of an austenite phase and suppressing the occurrence of leasing. From the viewpoint of obtaining such an effect, the Mn content is set to 0.45% or more.

However, when the Mn content exceeds 1.00%, the steel is excessively hardened and the rolling load during hot rolling increases. Further, the ductility of the steel sheet obtained after annealing of the cold-rolled sheet decreases.

Therefore, the Mn content is in the range of 0.45 to 1.00%. The lower limit of the Mn content is preferably 0.60%. The upper limit of the Mn content is preferably 0.75%, more preferably 0.70%.

P: 0.04% or less

P is an element that promotes grain boundary destruction due to grain boundary segregation. Therefore, it is preferable that the P content is less, and the upper limit is set to 0.04%. Preferably it is 0.03% or less. More preferably, it is 0.01% or less. The lower limit of the P content is not particularly limited, but excessive removal of P causes an increase in cost. Therefore, the lower limit of the P content is preferably 0.005%.

S: 0.010% or less

S is an element that exists in steel as a sulfide-based inclusion such as MnS and lowers ductility or corrosion resistance, and particularly, when the S content exceeds 0.010%, the adverse effect occurs remarkably. Therefore, it is preferable that the S content is as low as possible, and the upper limit of the S content is set to 0.010%. Preferably it is 0.007% or less. More preferably, it is 0.005% or less. The lower limit of the S content is not particularly limited, but excessive removal of S causes an increase in cost. Therefore, the lower limit of the S content is preferably 0.001%.

Cr: 16.0 to 18.0%

Cr is an element having an effect of improving corrosion resistance by forming a passivation film on the surface of a steel sheet. From the viewpoint of obtaining such an effect, the Cr content is 16.0% or more.

However, when the Cr content exceeds 18.0%, the amount of austenite phase generated during hot rolling decreases, and there is a fear that the unloading property may decrease.

Therefore, the Cr content is in the range of 16.0 to 18.0%. The upper limit of the Cr content is preferably 17.0%, more preferably 16.5%.

Al: 0.001 to 0.010%

Al, like Si, is an element that acts as a deoxidizing agent. From the viewpoint of obtaining such an effect, the Al content is set to 0.001% or more.

However, when the Al content exceeds 0.010%, Al-based inclusions, such as Al ₂ O ₃ , increase, which tends to cause a decrease in surface properties.

Therefore, the Al content is in the range of 0.001 to 0.010%. The lower limit of the Al content is preferably 0.002%. The upper limit of the Al content is preferably 0.007%, more preferably 0.005%.

N: 0.025 to 0.060%

Like C and Mn, N is an element effective in promoting the formation of an austenite phase during hot rolling and suppressing the occurrence of leasing. From the viewpoint of obtaining such an effect, the N content is set to 0.025% or more.

However, when the N content exceeds 0.060%, the ductility of the steel sheet obtained after the cold-rolled sheet annealing is significantly reduced. Further, the precipitation amount of Cr-based carbonitrides during hot rolling and hot-rolled sheet annealing becomes excessively large, and it becomes difficult to increase the average distance between Cr-based carbonitrides. For this reason, in the case of performing extension molding, the occurrence of cracks caused by connection of voids and fracture due to propagation cannot be prevented, and desired extension moldability cannot be obtained.

Therefore, the N content is in the range of 0.025 to 0.060%. The lower limit of the N content is preferably 0.030%, more preferably 0.040%. The upper limit of the N content is preferably 0.055%, more preferably 0.050%.

Ni: 0.05 to 0.60%

Ni is an element having an effect of promoting formation of an austenite phase, increasing the amount of austenite phase produced during hot rolling, and improving unloading properties. Moreover, Ni is an element effective also for improvement of corrosion resistance. From the viewpoint of obtaining such an effect, the Ni content is set to be 0.05% or more. However, when the Ni content exceeds 0.60%, the steel is excessively hardened and the formability is deteriorated. Therefore, the Ni content is in the range of 0.05 to 0.60%. The lower limit of the Ni content is preferably 0.10%. The upper limit of the Ni content is preferably 0.50%, more preferably 0.30%.

In addition, components other than the above are Fe and unavoidable impurities.

Next, the metal structure of the ferritic stainless steel sheet of the present invention will be described.

The metal structure of the ferritic stainless steel sheet of the present invention has a structure mainly composed of a ferrite phase, specifically, a ferrite phase of 90% or more in terms of volume ratio to the entire structure, and the remaining structure other than the ferrite phase is It becomes a structure which becomes 10% or less by volume ratio. Moreover, ferrite single phase may be sufficient. In addition, as the residual structure, a martensite phase is mainly mentioned, and the volume ratio of precipitates and inclusions is not included.

Here, the volume ratio of the ferrite phase is determined by preparing a test piece for cross-sectional observation from a stainless steel sheet, performing an etching treatment with a saturated picric acid-5% by mass hydrochloric acid aqueous solution after mirror polishing, and then at the position of 1/4 of the plate thickness. Observation with an optical microscope at a magnification of 100 times for 10 fields of view, and after discriminating the martensite phase and the ferrite phase from the shape of the metal structure, the volume ratio of the ferrite phase is determined by image processing, and the average value is calculated. .

In addition, in the metal structure of the ferritic stainless steel sheet of the present invention, as described above, it is important that the average distance between Cr-based carbonitrides having a diameter equivalent to a circle precipitated in the steel of 0.05 µm or more is 3.0 µm or more.

Here, the equivalent circle diameter is,

With respect to the digital photo (500 times magnification) of the Cr-based carbonitride that appeared in the metal structure of the above-described cross-sectional observation test piece, image processing was performed to measure the area of the Cr-based carbonitride,

The circle diameter calculated based on the assumption that the shape of the Cr-based carbonitride is a true circle from the measured area of the Cr-based carbonitride (=｛(4 × [the measured area of the Cr-based carbonitride])/π｝ ^0.5 )

Means.

Average distance between Cr-based carbonitrides with a circle equivalent diameter of 0.05 µm or more: 3.0 µm or more

If the average distance of Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more is lengthened to 3.0 µm or more, connection between voids is unlikely to occur even in the case of performing elongation molding that deforms under multiaxial stress. As a result, voids are formed. The occurrence and progress of microcracks resulting from the connection are suppressed

For this reason, the average distance of Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more is 3.0 µm or more. It is preferably 4.0 μm or more. In addition, the upper limit is not particularly limited, and is usually about 6.0 µm.

In addition, Cr-based carbonitrides with a circle equivalent diameter of less than 0.05 µm are not targeted. Since very fine Cr-based carbonitrides with a circle equivalent diameter of less than 0.05 µm have a small area in contact with the ferrite phase, which is a parental shape, press processing, etc. This is because even when plastic deformation is applied, voids are hardly generated at the interface between the ferrite phase and the Cr-based carbonitride, and thus the influence on the moldability, particularly the elongation moldability, can be almost neglected.

In addition, the Cr-based carbonitride here is a generic term for Cr carbide and Cr nitride. As Cr carbide, Cr ₂₃ C ₆ is mentioned, and as Cr nitride, Cr ₂ N is mentioned, for example. In addition, a part of Cr in the Cr carbide and Cr nitride is substituted with an element such as Fe or Mn to be included in the Cr-based carbonitride referred to herein.

In addition, targeting Cr-based carbonitrides with an equivalent circle diameter of 0.05 µm or more, voids that occur with an increase in the amount of deformation are mainly generated at the interface between a ferrite matrix and a Cr-based carbonitride having a circle equivalent diameter of 0.05 µm or more. This is because the distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more particularly affects the connection of the voids, and furthermore, the elongation formability.

In addition, the size of the Cr-based carbonitride is usually about 0.5 µm in a circle equivalent diameter.

In addition, the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more was measured as follows.

That is, on the rolled parallel cross section of the steel sheet, after mirror polishing, a metal structure is formed by etching with a saturated hydrochloric acid solution of picric acid, and a metal structure at a position of 1/4 of the plate thickness is photographed with an optical microscope at a magnification of 500 times. .

In addition, that the precipitate identified in the metal structure photograph is a Cr-based carbonitride can be confirmed by performing component analysis of the precipitate by an energy dispersive X-ray spectroscopy under a scanning electron microscope. Specifically, the peak of Cr in the element spectrum obtained from the precipitate by energy dispersive X-ray spectroscopy is higher than the peak of Cr in the element spectrum obtained from the matrix by the same method, and each element of the precipitate In the quantitative analysis value of each element calculated from the spectral intensity ratio of, when the main components of the precipitate were Cr, Fe, C and N, the precipitate can be judged as a Cr-based carbonitride.

Subsequently, in the obtained metal structure photograph, an arbitrary Cr-based carbonitride (hereinafter, also referred to as a reference carbonitride) having a circle equivalent diameter of 0.05 μm or more is selected, in the order of close distance from the reference carbonitride, and the circle equivalent diameter. Ten Cr-based carbonitrides (also referred to as target carbonitrides) of 0.05 µm or more are selected, and the distance (distance between the centers) between the reference carbonitride and each target carbonitride is measured on a metal structure photograph.

This measurement is carried out 20 times by arbitrarily changing the reference carbonitride, and the average distance between the Cr-based carbonitride is obtained by arithmetic average of the distances between all the measured reference carbonitrides and the target carbonitride.

In addition, the above-described measurement is not limited to a single ferrite grain, and may span grain boundaries. In addition, in order to carry out a representative measurement, each measurement point must be sufficiently separated from each other so that the reference carbonitride and the target carbonitride selected in the previous measurement do not become the reference carbonitride or the target carbonitride in other measurements. Choose.

As described above, the ferritic stainless steel sheet of the present invention has the above-described component composition and the average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more is 3.0 µm or more, based on the molding limit line (FLD). Thus, the minimum value of the maximum logarithmic deformation of the molding limit determined is set to 0.20 or more, preferably 0.23 or more, so that excellent elongation moldability can be obtained.

In addition, the plate thickness of the ferritic stainless steel sheet according to the embodiment of the present invention is not particularly limited, but is, for example, 0.8 to 2.0 mm.

Next, a method of manufacturing the ferritic stainless steel sheet of the present invention will be described.

In the ferritic stainless steel sheet of the present invention, a steel material having the above-described component composition is hot-rolled to form a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to annealing of the hot-rolled sheet at a heating temperature of 800 to 900°C and holding time for 1 hour or more. , The hot-rolled steel sheet is cold-rolled to form a cold-rolled steel sheet, and the cold-rolled steel sheet is subjected to a cold-rolled sheet annealing of a heating temperature of 800 to 900°C and a holding time of 5 to 300 seconds, and in the above cold-rolled sheet annealing, 500 It can manufacture by making the average heating rate at 20 degreeC/s or less in degreeC-heating temperature, and making it 10 degreeC/s or more in a heating temperature-500 degreeC average cooling rate.

That is, the ferritic stainless steel sheet according to the embodiment of the present invention is a ferritic stainless cold rolled annealed steel sheet.

First, molten steel composed of the above-described component composition is melted by a known method such as a converter, an electric furnace, and a vacuum melting furnace, and a steel material (slab) is obtained by a continuous casting method or an ingot-disintegration method.

Subsequently, the obtained steel material is preferably heated at 1100 to 1250°C for 1 to 24 hours, or after directly heating a high-temperature slab, hot rolling is performed on the steel material to obtain a hot rolled steel sheet. In addition, about the hot rolling conditions, it is good to follow a usual method.

Next, the obtained hot-rolled steel sheet is subjected to hot-rolled sheet annealing under the following conditions.

<Heating temperature of hot-rolled sheet annealing: 800 to 900°C, holding time: 1 hour or more>

The metal structure of the hot-rolled steel sheet is a metal structure in which a ferrite phase and a ferrite phase generated by decomposition of the austenite phase generated at high temperature are laminated in a layered form when the winding temperature during hot rolling is high, and the winding temperature during hot rolling is low. In this case, the ferrite phase and the austenite phase formed at high temperature are transformed to form a metal structure in which the formed martensite phase is layered.

In addition, in the vicinity of the ferrite phase generated by decomposition of the austenite phase when the coiling temperature is high, Cr-based carbonitrides precipitated due to the decomposition of the austenite phase are ubiquitous, and the distribution of Cr-based carbonitrides throughout the metal structure is It is uneven.

Further, the coiling temperature is not particularly limited, but when the coiling temperature is set to 450°C to 500°C, remarkable decrease in toughness of the hot-rolled steel sheet due to embrittlement at 475°C may occur. Therefore, it is preferable to set the coiling temperature to more than 500°C or less than 450°C. In addition, from the viewpoint of more easily obtaining a predetermined metal structure after hot-rolled sheet annealing, it is more advantageous to sufficiently precipitate Cr-based carbonitride at the stage after hot-rolling and before hot-rolled sheet annealing. For this reason, it is more preferable that the coiling temperature is set to 600°C or higher at which decomposition into the austenitic Cr-based carbonitride and ferrite phase is further accelerated.

The hot-rolled steel sheet having such a metal structure is subjected to hot-rolled sheet annealing at a temperature range of 800 to 900°C for 1 hour or more, whereby recrystallization and precipitation of Cr-based carbonitride occur in the metal structure, which is obtained after the hot-rolled sheet annealing. In the steel sheet, a metal structure in which Cr-based carbonitrides are sufficiently and uniformly dispersed in a single-phase ferrite structure is obtained.

Here, when the heating temperature of the hot-rolled sheet annealing is set to less than 800°C, aggregation and coarsening of Cr-based carbonitrides and solid solution to the ferrite phase are insufficient, and a predetermined metal structure cannot be obtained. Further, the recrystallization is insufficient, and the layered structure formed during hot rolling remains, particularly in the center of the plate thickness. For this reason, after annealing of the cold-rolled sheet, a non-uniform metal structure having remarkable whole grains is generated in the central portion of the sheet thickness, and there is a fear that the unrolling property may decrease.

On the other hand, when the hot-rolled sheet annealing temperature exceeds 900° C., the austenite phase is regenerated during the maintenance of the hot-rolled sheet annealing, and the Cr-based carbonitride precipitated in the hot rolling step is dissolved in the austenite phase. For this reason, in the metal structure of the steel sheet obtained after hot-rolled sheet annealing, Cr-based carbonitride cannot be sufficiently precipitated. Further, during cooling of the hot-rolled sheet annealing, a decomposition reaction between the ferrite phase and the Cr-based carbonitride occurs in the austenite phase. As a result, the metal structure after annealing of the hot-rolled sheet becomes a mixed structure of a ferrite phase generated by decomposition of the ferrite phase and the austenite phase, that is, a ferrite phase in which a large amount of Cr-based carbonitride is distributed around it, and the Cr-based carbonitride The distribution of is uneven. For this reason, even if cold-rolled sheet annealing is performed under predetermined conditions in a subsequent step, a region in which the average distance between Cr-based carbonitrides is insufficient is locally generated, and a predetermined elongation formability cannot be obtained.

Therefore, the heating temperature in the hot-rolled sheet annealing is in the range of 800 to 900°C. It is preferably in the range of 800 to 860°C.

In addition, when the holding time in the hot-rolled sheet annealing is less than 1 hour, precipitation of Cr-based carbonitride is insufficient, and even if cold-rolled sheet annealing is performed under predetermined conditions in the subsequent process, the average between Cr-based carbonitrides The distance cannot be sufficiently long, and a predetermined elongation moldability cannot be obtained.

Therefore, the holding time in the hot-rolled sheet annealing is set to 1 hour or more. It is preferably 3 hours or more, more preferably 5 hours or more. Moreover, although there is no particular limitation on the upper limit of the holding time, it is preferable to set it as 24 hours or less from a viewpoint of productivity.

Next, the steel sheet (hot-rolled annealed steel sheet) obtained after the hot-rolled sheet annealing is subjected to pickling as necessary, and cold-rolled to obtain a cold-rolled steel sheet. It is preferable to perform cold rolling at a reduction ratio of 50% or more from the viewpoint of extensibility, bendability, and shape correction. Further, the cold-rolled-cold-rolled sheet annealing may be repeated two or more times within a range that satisfies the cold-rolled sheet annealing conditions described later. Further, in order to improve the surface properties, the steel sheet obtained after the hot-rolled sheet annealing may be subjected to grinding or polishing.

The cold-rolled steel sheet thus obtained is subjected to cold-rolled sheet annealing under the following conditions.

<Average heating rate in 500°C to heating temperature of cold-rolled sheet annealing: 20°C/s or less>

Cold-rolled sheet annealing is a step for recrystallizing the rolled structure formed by cold rolling, and for sufficiently lengthening the average distance between Cr-based carbonitrides. In order to do so, the average heating rate at 500°C to a heating temperature is 20 It is important to make it below °C/s.

That is, if the average heating rate at 500°C to the heating temperature is decreased, the driving force for recrystallization decreases, so the temperature at which recrystallization starts is increased, and the dislocation or shear band introduced by cold rolling is maintained to a higher temperature.

In addition, in the high-temperature region close to the heating temperature, the agglomeration and coarsening of the Cr-based carbonitrides produced during annealing of the hot-rolled sheet (each Cr-based carbonitride increases while the volume ratio is almost constant, and the number density of the Cr-based carbonitride is increased. Small phenomenon) and solid solution to the ferrite phase occurs. This coagulation and coarsening is controlled by the diffusion of Cr, which is a major constituent element of Cr-based carbonitride. As described above, when the dislocation or shear zone is maintained to a high temperature, high-speed diffusion of Cr occurs through the dislocation or shear zone, thereby promoting aggregation and coarsening of the Cr-based carbonitride.

In addition, the reason why the solid solution of Cr-based carbonitride occurs in a high-temperature region close to the heating temperature is due to an increase in the upper limit (solubility limit) of C and N that can be dissolved in the ferrite phase. The average distance between Cr-based carbonitrides increases due to the effect of promoting aggregation and coarsening of these Cr-based carbonitrides and solid solution to the ferrite phase. That is, it becomes possible to increase the average distance between Cr-based carbonitrides by slowing the heating rate, specifically, controlling the average heating rate at 500°C to 20°C/s or less at the heating temperature.

On the other hand, if the average heating rate at 500°C to the heating temperature exceeds 20°C/s, the driving force of the ferrite phase recrystallization becomes excessively large, and the ferrite phase recrystallization occurs from a relatively low temperature region of the heating process, and is caused by cold rolling. The introduced dislocation or shear band is replaced with recrystallized grains in the low temperature region. As a result, the effect of promoting aggregation and coarsening of the Cr-based carbonitride is insufficient, and the average distance between the Cr-based carbonitrides in the steel sheet obtained after the cold-rolled sheet annealing is shortened, so that the desired lengthening formability cannot be obtained.

Therefore, the average heating rate at 500°C to the heating temperature is 20°C/s or less. It is preferably 15°C/s or less, and more preferably 12°C/s or less. In addition, the lower limit of the average heating rate is not particularly limited. However, if the heating rate is excessively slow, productivity is lowered, so it is preferable to set it to 1° C./s or more.

In addition, in the case of a continuous annealing method, for example, the control of the heating rate can be controlled by setting a furnace temperature or a plate speed of a continuous annealing line.

In addition, the reason why the temperature range to be controlled is 500°C or higher is because recovery or recrystallization does not occur in the temperature range below 500°C.

<Heating temperature of cold-rolled sheet annealing: 800 to 900°C, holding time: 5 to 300 seconds>

The upper limit (solubility limit) of C and N that can be dissolved in the ferrite phase increases as the temperature increases. In cold-rolled sheet annealing, heating temperature: 800 to 900°C and holding time: 5 to 300 seconds are used to dissolve part of the Cr-based carbonitride generated during hot-rolled sheet annealing in the ferrite phase and reduce the number density of Cr-based carbonitrides. Thus, the average distance between Cr-based carbonitrides can be lengthened.

For this reason, the heating temperature in the cold-rolled sheet annealing is 800 to 900°C, and the holding time is 5 to 300 seconds. Preferably, the heating temperature in the cold-rolled sheet annealing is 800 to 860°C, and the holding time is 15 to 180 seconds.

In addition, the holding time here is a residence time in a temperature range of a heating temperature ±10 degreeC.

Here, when the heating temperature is less than 800°C, the solid solution limit of C and N in the ferrite phase is not sufficiently large, and the amount of Cr-based carbonitride dissolved in the ferrite phase decreases, and the distance between the Cr-based carbonitride becomes short. . For this reason, the desired elongation moldability cannot be obtained. In addition, unrecrystallized grains remain, and the ductility is greatly reduced.

On the other hand, when the heating temperature exceeds 900°C, an austenite phase is generated during holding, and in subsequent cooling, the austenite phase is transformed into a martensite phase, and the steel sheet is remarkably hardened. Further, the metal structure of the final product plate becomes a two-phase structure of a ferrite phase and a martensite phase, and the plastic deformability is remarkably lowered, and the desired elongation formability is not obtained.

Further, when the holding time is less than 5 seconds, the solid solution of the Cr-based carbonitride in the ferrite phase in the holding is incomplete, the distance between the Cr-based carbonitrides is shortened, and the desired extension formability cannot be obtained. In addition, since non-recrystallized grains remain, the ductility is greatly reduced.

On the other hand, when the holding time exceeds 300 seconds, the crystal grains become remarkably coarse and the glossiness of the steel sheet decreases, which is not preferable from the viewpoint of surface quality.

<The average cooling rate in heating temperature of cold-rolled sheet annealing-500 degreeC: 10 degreeC/s or more>

In the above-described cooling after holding, re-precipitation of Cr-based carbonitride occurs. That is, during maintenance of the cold-rolled sheet annealing, the Cr-based carbonitride is dissolved in the ferrite matrix. Thereby, the solid solution C and N in the ferrite phase increase, become supersaturated with respect to the ferrite phase during cooling, and re-precipitate as a Cr-based carbonitride.

Therefore, from the viewpoint of increasing the average distance of the Cr-based carbonitride, in particular, by increasing the cooling rate in a temperature range of 500°C or higher, which is the precipitation temperature range of the Cr-based carbonitride, reprecipitation of the Cr-based carbonitride is suppressed. It becomes important to maintain a metal structure with a sufficiently long average distance between Cr-based carbonitrides formed before cooling.

Here, when the average cooling rate is less than 10° C./s, the solid solution C and N in the ferrite phase generated during maintenance of the cold-rolled sheet annealing cannot sufficiently suppress re-precipitation as Cr-based carbonitrides. The average distance is shortened, and the desired elongation moldability cannot be obtained.

Therefore, the average cooling rate in the heating temperature of the cold-rolled sheet annealing at 500°C is 10°C/s or more. It is preferably 15°C/s or more, and more preferably 20°C/s or more. In addition, the upper limit of the average cooling rate is not particularly limited, but since there is a risk that deformation may occur in the steel sheet when cooling is performed rapidly, the average cooling rate is preferably 200°C/s or less.

In addition, the cooling method is not particularly limited, and gas jet cooling, mist cooling, roll cooling, or the like can be used. Moreover, about the cold-rolled sheet annealing, BA annealing (bright annealing) may be performed in order to obtain higher gloss.

Then, after the above-described cold-rolled sheet annealing, the above-described ferritic stainless steel sheet is manufactured by performing pickling as necessary.

Example

Example 1

Components shown in Table 1 (The remainder is Fe and inevitable impurities) molten steel, respectively, by refining using a converter with a capacity: 150 ton and a vacuum oxygen decarburization (VOD) method, and then by continuous casting, width: 1000 mm, thickness: It was set as a 200 mm slab.

After heating the slab at 1200°C for 1 hour, as hot rolling, a 7-pass rough rolling using a reverse rolling mill consisting of a three-stage stand and a seven-pass finish using a one-way rolling machine consisting of a seven-stage stand Rolling was performed, and winding-up treatment was performed at about 750 degreeC, and it was set as the hot-rolled steel sheet of plate thickness: about 5.0 mm.

Next, these hot-rolled steel sheets were subjected to hot-rolled sheet annealing using the box annealing method under the conditions shown in Table 2, and then shot blasting treatment and descaling by pickling were performed on the surface.

The steel sheet thus obtained was cold-rolled to a thickness of 1.0 mm, and then cold-rolled sheet annealing was performed under the conditions shown in Table 2. In addition, cooling after holding was performed by gas jet cooling or mist cooling. Further, after the cooling was completed, a descaling treatment by pickling was performed.

Here, the holding time in Table 2 is the residence time in the temperature range of the heating temperature ± 10 degreeC.

In addition, the average heating rate described in Table 2 is an average heating rate from 500°C to reaching the heating temperature. In addition, the average cooling rate described in Table 2 is the average cooling rate until reaching the heating temperature to 500°C.

For the steel sheet thus obtained, the identification of the metal structure and the measurement of the volume fraction of ferrite, and the measurement of the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more were performed.

Here, identification of the metal structure and measurement of the volume ratio of ferrite were performed by the method described above. That is, a test piece for cross-sectional observation was prepared from the obtained steel sheet, and after etching treatment with a saturated hydrochloric acid solution of picric acid was performed, observation with an optical microscope was performed at a magnification of 100 times for 10 fields of view at a position of 1/4 of the plate thickness, After distinguishing the martensite phase and the ferrite phase from the shape of the metal structure, the volume ratio of the ferrite phase was determined in each field by image processing, and the average value was taken as the volume fraction of the ferrite phase. In addition, the volume ratio of precipitates and inclusions is excluded.

Further, the average distance between Cr-based carbonitrides having an equivalent circle diameter of 0.05 µm or more was also measured by the method described above.

These results are similarly shown in Table 2. In addition, for reference, Figs. 1 and 2 show photographs of metal structures used for measurement of the average distance between Cr-based carbonitrides for No. 1 and No. 12 in Table 2.

Moreover, (1) evaluation of elongation moldability, and (2) evaluation of corrosion resistance were performed by the following method. The evaluation results are listed in Table 2.

(1) Evaluation of elongation formability

With the rolling parallel direction, the rolling 45° direction, and the rolling right angle direction of the obtained steel sheet as the maximum logarithmic deformation directions, respectively, a forming test according to ISO12004-2:2008 was performed, and a forming limit line diagram (FLD) was created.

Specifically, a scribed circle having a diameter of 5 mm is marked on the surface of the steel sheet so that the distance between ratings is 1 mm, and a molding test under various conditions, that is,

・For the molding limit under short stress, a tensile test using a JIS No. 5 tensile test piece,

-About the molding limit in the state of plane deformation, a bulging test using a test piece to which a bead was applied on a circumference after shearing to 130 mm in width and length,

For the molding limit under the same biaxial stress, a bulging test using a blank test piece in a circular shape,

For the molding limit under unequal biaxial stress, a bulging test using elliptical blank specimens with various elliptic ratios

Each was carried out, and the test pieces before and after each test were photographed, respectively. Subsequently, the amount of change in the shape of the scribed circle of the test piece before and after each test was quantitatively measured by image processing of a photograph, and the deformation imparted by each test was measured to create a shaping limit line (FLD).

The minimum value of the maximum logarithmic deformation of the molding limit was calculated from the obtained molding limit line (FLD), and the case where the minimum value of the maximum logarithmic deformation was 0.20 or more was evaluated as pass (○), and the case less than 0.20 was evaluated as reject (×).

(2) Evaluation of corrosion resistance

From the obtained steel sheet, a test piece of 60 x 100 mm was taken out, and the surface was polished with #600 emery paper. Thereafter, the cross-section of the test piece was sealed, and subjected to a salt spray cycle test specified in JIS H 8502.

Here, in the salt spray cycle test, salt spray (35° C., 5 mass% NaCl, spray time: 2 hours) → drying (60° C., 40% relative humidity, holding time: 4 hours) → wet (50° C., relative humidity) ≥ 95%, holding time: 2 hours) was 1 cycle, and 8 cycles were performed.

Take a picture of the surface of the test piece after the salt spray cycle test, measure the rust area on the surface of the test piece by image analysis, and calculate the rust area rate from the ratio with the total area of the test piece surface ((Rust area on the surface of the test piece/test piece The total area of the surface) x 100 (%)) was calculated.

And, the case where the calculated rust area ratio was 10% or less was evaluated as pass (◎, particularly excellent), more than 10% and 25% or less as pass (○), and the case of more than 25% as rejection (x).

In all of the invention examples, excellent elongation moldability and excellent corrosion resistance were obtained.

In particular, in No.6 containing 0.58% Ni (steel A6) and No.8 containing 17.8% Cr (steel A8), the rust generation area rate in the salt spray cycle test was 10% or less (◎) Thus, further excellent corrosion resistance was obtained.

On the other hand, comparative examples No.12 (steel B1) and No.13 (steel B2) have appropriate production conditions, but since the C content and N content exceed the appropriate ranges, respectively, the precipitation amount of Cr-based carbonitride is excessive. As a result, the average distance between Cr-based carbonitrides could not be sufficiently secured, and desired elongation moldability was not obtained.

In No. 14 (steel B3), since the Si content exceeded the appropriate range, the steel sheet was hardened and the plastic deformability was lowered, and the desired elongation formability was not obtained.

In No. 15 and No. 16, since the average heating rate of the cold-rolled sheet annealing exceeded an appropriate range, the average distance between Cr-based carbonitrides could not be sufficiently secured, and desired elongation moldability was not obtained.

In No.17 and No.18, since the heating temperature of the cold-rolled sheet annealing exceeds the appropriate range, an austenite phase is generated during maintenance of the cold-rolled sheet annealing, and is transformed from an austenite phase to a martensite phase in cooling after holding. , The steel plate was remarkably hardened. Further, since the metal structure of the final product sheet became a two-phase structure composed of a ferrite phase and a martensite phase, the plastic deformation ability of the steel sheet was remarkably lowered. For this reason, the desired elongation moldability was not obtained.

In No.19 and No.20, since the heating temperature of the cold-rolled sheet annealing is less than the appropriate range, the average distance between Cr-based carbonitrides cannot be sufficiently secured, and a metal structure in which non-recrystallized grains remain. Desired extension moldability was not obtained.

In No. 21 and No. 22, since the holding time of the cold-rolled sheet annealing is less than the appropriate range, the average distance between Cr-based carbonitrides cannot be sufficiently secured, and a metal structure in which non-recrystallized grains remain. Desired extension moldability was not obtained.

In No.23 and No.24, since the cooling rate of cold-rolled sheet annealing is less than the appropriate range, a large amount and finely re-precipitated Cr-based carbonitrides during the cooling, so that the average distance between Cr-based carbonitrides can be sufficiently secured. Could not be obtained, and the desired elongation moldability was not obtained.

In No.28, since the heating temperature of the hot-rolled sheet annealing is less than the appropriate range, the aggregation and coarsening of Cr-based carbonitrides in hot-rolled sheet annealing and the solid solution to the ferrite phase are insufficient, and distribution of Cr-based carbonitrides There was a non-uniformity of. For this reason, a region in which the average distance between Cr-based carbonitrides is not sufficient was formed locally, and desired elongation moldability was not obtained.

In No.29, since the heating temperature of the hot-rolled sheet annealing exceeds the appropriate range, the austenite phase is regenerated in the hot-rolled sheet annealing, and as a result, the distribution of Cr-based carbonitrides in the metal structure after the hot-rolled sheet annealing is uneven. Occurred. For this reason, a region in which the average distance between Cr-based carbonitrides is not sufficient was formed locally, and desired elongation moldability was not obtained.

In No.30, since the holding time of the hot-rolled sheet annealing is less than the appropriate range, the aggregation and coarsening of Cr-based carbonitrides in hot-rolled sheet annealing and the solid solution to the ferrite phase are insufficient, and distribution of Cr-based carbonitrides There was a non-uniformity of. For this reason, a region in which the average distance between Cr-based carbonitrides is not sufficient was formed locally, and desired elongation moldability was not obtained.

Industrial availability

The ferritic stainless steel sheet of the present invention is particularly advantageous for applications requiring high elongation formability during press molding, for example, to exterior members, kitchen utensils, and tableware.

Claims (2)

By mass%,
C: 0.025 to 0.050%,
Si: 0.10 to 0.40%,
Mn: 0.45 to 1.00%,
P: 0.04% or less,
S: 0.010% or less,
Cr: 16.0 to 18.0%,
Al: 0.001 to 0.010%,
N: 0.025 to 0.060% and
Ni: 0.05 to 0.60%
And the balance has a component composition consisting of Fe and unavoidable impurities,
The average distance between Cr-based carbonitrides having a circle equivalent diameter of 0.05 µm or more is 3.0 µm or more,
Ferritic stainless steel sheet with a minimum value of 0.20 or more of the maximum logarithmic deformation of the forming limit based on the forming limit diagram. A steel material having the component composition described in claim 1 is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is subjected to annealing of a hot-rolled sheet at a heating temperature of 800 to 900°C and a holding time of 1 hour or more, and then cold-rolled to form a cold-rolled steel sheet. Then, the cold-rolled steel sheet was annealed with a heating temperature of 800 to 900°C and a holding time of 5 to 300 seconds,
In the above cold-rolled sheet annealing, ferrite, wherein the average heating rate at 500°C to the heating temperature is 20°C/s or less, and the average cooling rate at the heating temperature to 500°C is 10°C/s or more. Method of manufacturing a series stainless steel plate.

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